Abstracts

Rationale: Ictal events originating in the temporal lobe and electrical stimulation of the temporal lobe sites occasionally evoke vivid and compelling visual images reported by patients. The neuronal mechanisms underlying these observations remain elusive. Here we sought to identify neurons in the medial temporal lobe that respond when patients conjure visual images in the absence of visual stimulation. Methods: Recordings were made from 276 single neurons via intracranial depth electrodes in 9 patients with intractable epilepsy undergoing invasive evaluation for surgery while the patients were presented with visual stimuli, and while they subsequently formed visual images in their mind's eye. Visual stimuli were of different categories such as faces, household objects, animals and spatial layouts. Results: 22 (8%) single neurons in the hippocampus, amygdala, entorhinal cortex and parahippocampal gyrus had selectively altered firing rates depending on the category of the stimulus the subjects were imagining. Of these neurons, 16 (73%) were also selective during vision and 14/16 (88%) responded to the same stimulus categories during vision and visual imagery. Conclusion: These results suggest a neuronal mechanism of evoking visual images without concommitant retinal stimulation. It is likely that neuronal networks in the temporal lobe active during actual vision are reactivated during ictal- and stimulation-evoked visual hallucinations in epilepsy patients. Supported by grants from NIH (NS33221 and NS02808) and the John Fetzer Institute.